[leephillips]

This part was fascinating:

“The problem is how FTZ actually implemented on most hardware: it is not set per-instruction, but instead controlled by the floating point environment: more specifically, it is controlled by the floating point control register, which on most systems is set at the thread level: enabling FTZ will affect all other operations in the same thread.

“GCC with -funsafe-math-optimizations enables FTZ (and its close relation, denormals-are-zero, or DAZ), even when building shared libraries. That means simply loading a shared library can change the results in completely unrelated code, which is a fun debugging experience.”